Originally Posted by DallasSoxFan

He wasn't saying a laptop battery. he was saying a UPS - Uninterruptable power supply. It is basically a small car battery (lead-acid) hooked up to a power strip. Power goes down, battery goes up. It would run a full-strength angle grinder with a diamond blade anywhere from 20 minutes for the smallest UPS's to hours for even moderate sized ones.

Actually, this is a pretty great idea, you'd probably get far more torque out of a corded angle grinder on a UPS than on a cordless 18volt.